A gate drive circuit in a semiconductor switching device (or a circuit which drives a semiconductor switching device) is a circuit which drives a gate terminal of the switching device (hereinafter the semiconductor switching device is also simply referred to as “switching device”). Such a gate drive circuit controls ON/OFF of the semiconductor switching device by applying a gate voltage to a gate terminal of, for example, a high-pressure resistant switching device such as an insulated gate bipolar transistor (IGBT) called a power semiconductor. A general gate drive circuit includes an output unit made of a P-type transistor and an N-type transistor. The P-type transistor operates when the switching device is turned ON from OFF and the N-type transistor operates when the switching device is turned OFF from ON. Specifically, when the switching device is turned OFF from ON, the gate current of the switching device is extracted.
In the above-described gate drive circuit, a reference voltage of the semiconductor switching device, that is the reference voltage on the output side of the gate driving circuit, is very high. Therefore, it is required to isolate a direct-current component (a signal ground) between the input side (primary side) of the gate drive to which a control signal is input and the output side (secondary side) of the gate drive circuit which drives the switching device. Such an electronic circuit element capable of isolating the direct-current component between the primary side and the secondary side is called a DC isolation device (or wireless signal transmitter). This is an element necessary to drive the switching device. Furthermore, the above-described electronic circuit element having the DC isolation function is used for isolating the logic ground and the RF ground, and is also called a digital isolator (Patent Literature (PTL) 1). Particularly, to drive a power semiconductor switching device, an external isolated power supply is required which requires a very large gate drive circuit also called a gate drive system. Therefore, if it is allowed not only to isolate a gate signal but also to supply isolated power to the gate, the external isolated power supply is no longer required and the gate isolating circuit can be miniaturized.
As a signal transmission circuit having the DC isolation function, as shown in FIG. 7, a power (signal) transmission device using an open-ring electromagnetic resonance coupler as a wireless signal transmitter has been proposed (for example, see PTL 2).